Combining autoclave and LCWM reactor studies to shed light on the kinetics of glucose oxidation catalyzed by doped molybdenum-based heteropoly acids

Abstract

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we investigated the influence of Nb-and Ta-doping on Mo-based Keggin-polyoxometalates for both reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose oxidation kinetics by Ta-and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced from around 80 kJ mol À1 for the classical HPA-Mo to 61 kJ mol À1 for the Ta-and 55 kJ mol À1 for the Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show significant differences between the different catalysts.

Highly Efficient Conversion of Carbohydrates into Formic Acid with a Heterogeneous MgO Catalyst at Near-Ambient Temperatures

“…For the first 60 min, the kinetic data were recorded and fitted by the Weibull model normalc glucose normalc 0 , glucose = exp [ − ( kt ) n ] where t is the reaction time, k is the reaction rate constant, and n is the shape constant. ,, After the determination of k values, the activation energy ( E a ) for glucose conversion was calculated by the Arrhenius law k = k 0 × exp [ − E A RT ] where k 0 is the frequency factor, R is the gas constant (8.314 J·mol –1 ·K –1 ), and T is the temperature . The slope was obtained as activation energy by fitting the curve.…”

“…where t is the reaction time, k is the reaction rate constant, and n is the shape constant. 26,32,46 After the determination of k values, the activation energy (E a ) for glucose conversion was calculated by the Arrhenius law…”

“…Hitherto, two general chemocatalytic oxidation strategies have been developed to produce FA from carbohydrate biomass. For oxidant O 2 , vanadium-containing catalysts, such as NaVO 3 , VOSO 4 , or polyoxometalates (POMs), have shown extraordinary activity due to the variable valence state of vanadium. − In particular, POMs are promising catalysts for glucose oxidation within the redox cycle because of their strong Brønsted acidity and high oxidizability. − However, issues such as a long reaction period (1–2 days) and high pressure of O 2 (20–60 bars) cannot be alleviated, as well as the inevitable problem of further decomposition from FA to CO 2 . As a robust oxidant, hydrogen peroxide (H 2 O 2 ) is often utilized to promote the oxidative decomposition of glucose into FA and always catalyzed by homogeneous bases (KOH or NaOH) at elevated temperature, in which considerable energy input and a sophisticated reaction system are needed. − Very recently, we reported that FA could be obtained from monosaccharides with an excess yield of 91.3% via the catalysis of LiOH near ambient temperature, which was achieved by accelerating • OH radical production from H 2 O 2 .…”

Valorization of secondary feedstocks from the agroindustry by selective catalytic oxidation to formic and acetic acid using the OxFA process

“…In this context, one attractive approach for valorization of organic residues is the transformation of lignocellulosic biomass into formic acid and acetic acid via metal catalysts (see Fig. 1a and b) [23][24][25][26][27][28], the so-called OxFA process [29]. The OxFA process runs under moderate temperature conditions (< 373 K) with oxidants such as air or oxygen.…”

Switchable Catalytic Polyoxometalate-Based Systems for Biomass Conversion to Carboxylic Acids

“…The whole framework structure has a typical size of 1–4 nm and contains typically 2–368 metal centers. − The catalytic activity is mostly introduced by substituting some of the framework metals with heterometals from the s-, p-, d-, or f-block. For example, the substitution of molybdenum atoms in the structure [PMo 12 O 40 ] 3 – with easily reducible heterometals like vanadium, niobium, or tantalum results in shifting their reactivity from acidic to redox-dominance and compounds with the general empirical formula [PV n Mo 12– n O 40 ] (3+ n )– . − Depending on the degree of substitution n , heteropolyanions of this structure are abbreviated as HPA- n . Wells–Dawson (WD) POMs have the general formula [X 2 M 18 O 62 ] n − and consist of two trilacunary Keggin structures (XM 9 n – ) linked in a corner-sharing manner.…”